PROTOCOL

The AEMP protocol is comparatively simple, and consists of three phases which are symmetrical for both sides: greeting (followed by optionally switching to TLS mode), authentication and packet exchange.

The protocol is designed to allow both full-text and binary streams.

The greeting consists of two text lines that are ended by either an ASCII CR LF pair, or a single ASCII LF (recommended).

GREETING

All the lines until after authentication must not exceed 4kb in length, including line delimiter. Afterwards there is no limit on the packet size that can be received.

First Greeting Line

The first line contains strings separated (not ended) by ; characters. The first five strings are fixed by the protocol, the remaining strings are KEY=VALUE pairs. None of them may contain ; characters themselves (when escaping is needed, use %3b to represent ; and %25 to represent %)-

The fixed strings are:

protocol identification

The constant aemp to identify this protocol.

protocol version

The protocol version supported by this end, currently 1. If the versions don't match then no communication is possible. Minor extensions are supposed to be handled through additional key-value pairs.

the node ID

This is the node ID of the connecting node.

the acceptable authentication methods

A comma-separated list of authentication methods supported by the node. Note that AnyEvent::MP supports a hex_secret authentication method that accepts a clear-text password (hex-encoded), but will not use this authentication method itself.

The receiving side should choose the first authentication method it supports.

the acceptable framing formats

A comma-separated list of packet encoding/framing formats understood. The receiving side should choose the first framing format it supports for sending packets (which might be different from the format it has to accept).

The remaining arguments are KEY=VALUE pairs. The following key-value pairs are known at this time:

provider=<module-version>

The software provider for this implementation. For AnyEvent::MP, this is AE-0.0 or whatever version it currently is at.

peeraddr=<host>:<port>

The peer address (socket address of the other side) as seen locally.

tls=<major>.<minor>

Indicates that the other side supports TLS (version should be 1.0) and wishes to do a TLS handshake.

Second Greeting Line

After this greeting line there will be a second line containing a cryptographic nonce, i.e. random data of high quality. To keep the protocol text-only, these are usually 32 base64-encoded octets, but it could be anything that doesn't contain any ASCII CR or ASCII LF characters.

The two nonces must be different, and an aemp implementation must check and fail when they are identical.

Example of a nonce line (yes, it's random-looking because it is random data):

TLS handshake

If, after the handshake, both sides indicate interest in TLS, then the connection must use TLS, or fail to continue.

Both sides compare their nonces, and the side who sent the lower nonce value ("string" comparison on the raw octet values) becomes the client, and the one with the higher nonce the server.

AUTHENTICATION PHASE

After the greeting is received (and the optional TLS handshake), the authentication phase begins, which consists of sending a single ;-separated line with three fixed strings and any number of KEY=VALUE pairs.

The three fixed strings are:

the authentication method chosen

This must be one of the methods offered by the other side in the greeting.

Note that all methods starting with tls_ are only valid iff TLS was successfully handshaked (and to be secure the implementation must enforce this).

The currently supported authentication methods are:

cleartext

This is simply the shared secret, lowercase-hex-encoded. This method is of course very insecure if TLS is not used (and not completely secure even if TLS is used), which is why this module will accept, but not generate, cleartext auth replies.

hmac_md6_64_256

This method uses an MD6 HMAC with 64 bit blocksize and 256 bit hash, and requires a shared secret. It is the preferred auth method when a shared secret is available.

First, the shared secret is hashed with MD6:

key = MD6 (secret)

This secret is then used to generate the "local auth reply", by taking the two local greeting lines and the two remote greeting lines (without line endings), appending \012 to all of them, concatenating them and calculating the MD6 HMAC with the key:

This type is only valid iff TLS was enabled and the TLS handshake was successful. It has no authentication data, as the server/client certificate was successfully verified.

This authentication type is somewhat insecure, as it allows a man-in-the-middle attacker to change some of the connection parameters (such as the framing format), although there is no known attack that exploits this in a way that is worse than just denying the service.

By default, this implementation accepts but never generates this auth reply.

tls_md6_64_256

This type is only valid iff TLS was enabled and the TLS handshake was successful.

and lowercase-hex encodes the result and sends it as authentication data. No shared secret is required (authentication is done by TLS). The checksum exists only to make tinkering with the greeting hard.

the authentication data

The authentication data itself, usually base64 or hex-encoded data, see above.

the framing protocol chosen

This must be one of the framing protocols offered by the other side in the greeting. Each side must accept the choice of the other side, and generate packets in the format it chose itself.

The shared secret in use was 8ugxrtw6H5tKnfPWfaSr4HGhE8MoJXmzTT1BWq7sLutNcD0IbXprQlZjIbl7MBKoeklG3IEfY9GlJthC0pENzk.

SIMPLE HANDSHAKE FOR NON-PERL NODES

Implementing the full set of options for handshaking can be a daunting task.

If security is not so important (because you only connect locally and control the host, a common case), and you want to interface with an AEMP node from another programming language, then you can also implement a simplified handshake.

For example, in a simple implementation you could decide to simply not check the authenticity of the other side and use cleartext authentication yourself. The the handshake is as simple as sending three lines of text, reading three lines of text, and then you can exchange JSON-formatted messages:

The nodename should be unique within the network, preferably unique with every connection, the <nonce> could be empty or some random data, and the hexencoded secret would be the shared secret, in lowercase hex (e.g. if the secret is "geheim", the hex-encoded version would be "67656865696d").

Note that apart from the low-level handshake and framing protocol, there is a high-level protocol, e.g. for monitoring, building the mesh or spawning. All these messages are sent to the node port (the empty string) and can safely be ignored if you do not need the relevant functionality.

USEFUL HINTS

Since taking part in the global protocol to find port groups is nontrivial, hardcoding port names should be considered as well, i.e. the non-Perl node could simply listen to messages for a few well-known ports.

Alternatively, the non-Perl node could call a (already loaded) function in the Perl node by sending it a special message:

["", "Some::Function::name", "myownport", 1, 2, 3]

This would call the function Some::Function::name with the string myownport and some additional arguments.

MONITORING

Monitoring the connection itself is transport-specific. For TCP, all connection monitoring is currently left to TCP retransmit time-outs on a busy link, and TCP keepalive (which should be enabled) for idle connections.

This is not sufficient for listener-less nodes, however: they need to regularly send data (30 seconds, or the monitoring interval, is recommended), so TCP actively probes.

Future implementations of AnyEvent::Transport might query the kernel TCP buffer after a write timeout occurs, and if it is non-empty, shut down the connections, but this is an area of future research :)

NODE PROTOCOL

The transport simply transfers messages, but to implement a full node, a special node port must exist that understands a number of requests.

If you are interested in implementing this, drop us a note so we finish the documentation.